Millimeter wave radiation systems are commonly used in security settings to identify high risk objects and materials by scanning a scene and using radiation reflected by or emitted by objects within the scene to determine the identity of various objects. Typically such systems produce an image of the radiometric brightness temperature of the scene at frequencies between around 30 GHz up to around 300 GHz. Many such systems tend to be passive and operate using a scanning mechanism, as described below.
One such system, as disclosed in U.S. Pat. No. 7,271,899 scans a receive beam of radiation from two or more points on a scene (typically a subject such as a person). A comparison is made between the radiometric brightness temperature of the points measured to determine any differences, which are subsequently used to detect the presence of an anomaly that may pose a security threat, such as a weapon or explosive device. In the case where two measurement points on a target are used, A beam of radiation is scanned sequentially from the two points using an optical system that passes the radiation through a two sector prism (see FIG. 15 of '899) after forming it into a parallel beam, where each sector produces a receive-beam in a given direction. The parallel beam may be formed by collection optics such as an afocal telescope, alternatively the prism may be located between the scene and the collection optics where the radiation is substantially parallel. This prism comprises a shallow cone sector structure and a shallow inverted cone sector structure, and is rotated about a central axis such that the beam passes through the prism off-axis close to the edge of the prism whilst it rotates. In order to scan a scene the two (or more) points on the target may be moved in an approximate square wave pattern, up, across, down, across, up, across and so on to cover the entire scene, which may typically be performed manually by an operator commanding a pan and tilt head. US'899 also discloses a rotating prism (see FIG. 7), which produces a circular scan of a beam on the target.
Although highly effective a manual scan is time-consuming, since the operator must scan the scene whilst interpreting the sensor response at each position scanned. For example, an entire torso must be scanned and any resulting alarm indications examined. This reduces the efficiency of the overall process, which may cause issues in high traffic areas requiring such scans.
One alternative is to use a system generating multiple beams in a vertical line, so that several beams at once pass over the scene. This means that an operator merely has to scan across the scene horizontally, rather than also vertically. Such a linear beam system may be produced by using a prism with several shallow cone sectors and shallow inverted cone sectors each of different opening angles, so that when the prism is rotated several beams are generated. So the prism itself is divided into zones, each zone a different cone sector, such that each zone is responsible for the formation of an individual beam. Although this arrangement offers several advantages over a two-beam system, it does have the disadvantage that there is an increasing proportion of dead time (time where measurements are not taking place, which occurs when the beam is cut by the transition between two zones) in the scan, as the number of zones increases. This reduces integration time and hence signal to noise ratio. This is not proportional to the number of beams formed. For instance, in a known implementation the reduction in signal to noise ratio for four beams is, for example, a factor of 1.95 compared to a two beam, approach or single beam switched between two different directions. For six beams the reduction is 2.83, and more than six beams becomes impractical due to the requirements placed on the size of the prism. In addition the response of the receiving element is highly dependent on the vertical position of the target area in the vertical line consisting of the multiple beams. For example, a target centred on a beam will produce a different response to one centred on the mid-point between two adjacent beams. This can create issues in determining the most appropriate detection algorithm to use to ensure reliability, and so may decrease performance in terms of hit rate and false alarm rate. Using improved sampling by increasing the number of beams formed that cover a given vertical angle may aid in reducing algorithm-related issues but the increase in the number of beams causes the integration time and signal to noise ratio to worsen yet further. There is therefore a need to be able to find a solution that overcomes the issues in sampling without detriment to the signal quality.